@kity yes I love this! also the other day I had an issue with one package in the current release-19.03 nixpkgs so I built the nix-shell environment I needed with a nixpkgs version that was a few days old, where Hydra had succeeded at building what I needed, and yet I didn't have to touch anything else on my system: https://github.com/jameysharp/reader-py/blob/master/shell.nix